"So a lot of the sites and alignments are based on views today without trees. Surely many sites would have been surrounded by trees in Ireland/Britain and these views would be obstructed.
Any thoughts?"

But also the reverse could apply. What we see as a possible connection would be an obvious one if there was a clear avenue to look along. As has been said though, it requires establishing by research. A pretty detailed study of prehistoric vegetation in the Kennett valley is on the net.